/Engine/Source/Runtime/Experimental/Iris/Core/Public/Iris/Serialization/NetSerializerDelegates.h
Pre freeze can be called before there are any serializers registered. At this point it's fine to register custom serializers for structs, but if you need a descriptor for a struct as a helper for your serializer you need to wait until post freeze.
virtual void OnPreFreezeNetSerializerRegistry()